Three-dimensional (3D) optical imaging systems, hereinafter referred to as “3D cameras”, that are capable of providing distance measurements to objects and points on objects that they image, are used for many different applications. Among these applications are profile inspections of manufactured goods, CAD verification, robot vision, geographic surveying, and imaging objects selectively as a function of distance.
Some 3D cameras provide simultaneous measurements to substantially all points of objects in a scene they image. Generally, these 3D cameras comprise a light source, typically comprising an array of edge emitting laser diodes, which is controlled to provide pulses of light for illuminating a scene being imaged, and a gated imaging system for imaging light from the light pulses that is reflected from objects in the scene. The gated imaging system comprises a camera having a photosensitive surface, hereinafter referred to as a “photosurface”, such as a CCD or CMOS photosurface and a gating means for gating the camera open and closed, such as an optical shutter or a gated image intensifier. The reflected light is registered on pixels of the photosurface of the camera only if it reaches the camera when the camera is gated open.
To image a scene and determine distances from the camera to objects in the scene, the light source is generally controlled to radiate a train of light pulses to illuminate the scene. For each radiated light pulse in the train, following an accurately determined delay from the time that the light pulse is radiated, the camera is gated open for a period hereinafter referred to as a “gate”. Light from the light pulse that is reflected from an object in the scene is imaged on the photosurface of the camera if it reaches the camera during the gate. Since the time elapsed between radiating a light pulse and the gate that follows it is known, the time it took imaged light to travel from the light source to the reflecting object in the scene and back to the camera is known. The time elapsed is used to determine the distance to the object.
In some “gated” 3D cameras, only the timing between light pulses and gates is used to determine distance from the 3D camera to a point in the scene imaged on a pixel of the photosurface of the camera. In others, an amount of light registered by the pixel during the time that the camera is gated open is also used to determine the distance. The accuracy of measurements made with these 3D cameras is a function of the rise and fall times of the light pulses and their flatness, and how fast the cameras can be gated open and closed.
Gated 3D cameras and examples of their uses are found in European Patent EP1214609 and in U.S. Pat. Nos. 6,057,909, 6,091,905, 6,100,517, 6,327,073, 6,331,911, 6,445,884, and 6,794,628, the disclosures of which are incorporated herein by reference. A 3D camera using a pulsed source of illumination and a gated imaging system is described in “Design and Development of a Multi-detecting two Dimensional Ranging Sensor”, Measurement Science and Technology 6 (September 1995), pages 1301-1308, by S. Christie, et al., and in “Range-gated Imaging for Near Field Target Identification”, Yates et al, SPIE Vol. 2869, p 374-385 which are herein incorporated by reference. Another 3D camera is described in U.S. Pat. No. 5,081,530 to Medina, which is incorporated herein by reference. A 3D camera described in this patent registers energy in a pulse of light reflected from a target that reaches the camera's imaging system during each gate of a pair of gates. Distance to a target is determined from the ratio of the difference between the amounts of energy registered during each of the two gates to the sum of the amounts of energy registered during each of the two gates.
R&D efforts to enhance accuracy of measurements provided by 3D cameras, are typically invested in developing methods and devices for reducing rise times, fall times, and widths of light pulses transmitted to illuminate a scene and corresponding gates during which light reflected from the pulses by the scene is imaged.